Wheat septoria results show further fungicide efficacy shifts

With septoria tritici disease resistance ratings eroding in some winter wheat varieties this year, there may be additional nervousness about disease control in 2022 – despite new options in the Recommended Lists (RL). With declines in fungicide efficacy a further concern, people eagerly awaited the latest fungicide performance update at this year’s Agronomists’ Conference.

This article outlines the key conclusions from the latest fungicide performance results, delivered by ADAS’ Jonathan Blake at the AHDB Agronomists’ Conference (7 December 2021).

Running since 1994, our fungicide performance work provides high-quality, independent information on the efficacy of fungicides against key diseases in wheat, barley and oilseed rape.

Knowledge of the potential power of an active ingredient (or product) is essential in disease management. First and foremost, it will help you gauge the potential return on your spray investments. Critically, it can be used to develop effective fungicide programmes that balance the need to control disease with the need to protect chemistry from fungicide resistance.

In theory, any pathogen can develop resistance to fungicides. However, the risk is not the same in all pathogens. In wheat, septoria tritici is currently of greatest concern.

Septoria populations have become less sensitive to azoles (DMIs). Since 2001, the fungicide performance team has used prothioconazole (full dose) to monitor septoria tritici control efficacy (Figure 1). Despite year-on-year variation, the gradual efficacy loss is clear – with 2021 registering the poorest control, so far.

However, the situation is complex. The loss of sensitivity is due to various mutations, with the presence of these mutations highly variable across the UK. There is also a relatively large variation in performance of active ingredients in this group. The addition of mefentrifluconazole (a relatively new azole) data to the chart in 2021 illustrates this point, with performance recorded at levels not observed with prothioconazole for over a decade.

Figure 1. DMI (azole) field activity against septoria tritici (2001–21) based on the mean control achieved by a full dose.

Following a period of relative stability (2008–16), monitoring started to detect significant shifts in sensitivity to SDHIs in 2017 (Figure 2). Fluxapyroxad has been used to monitor the trends (at a range of doses). The recorded full-dose control level in 2021 was around half the level achieved in 2016. As SDHIs show cross-resistance with other SDHIs, the efficacy data provides a warning for all users of SDHI chemistry. Like azoles, there appears to be year-on-year and site-to-site variability in control.

Figure 2. SDHI (fluxapyroxad) activity against septoria tritici (2016–21) at a range of doses.

Fungicide performance for wheat

• Despite the changes to efficacy, it is possible to achieve good control with a well-designed fungicide programme
• For septoria tritici, Univoq and Revystar XE showed the highest level of protectant activity, with performance (on disease levels) for these two products similar
• For yellow rust, all mixtures performed well, with Elatus Era particularly effective (however, spray timing is particularly critical for management of this disease)
• For brown rust, mefentrifluconazole and SDHIs were highly active – fenpicoxamid and prothioconazole added reasonable levels of activity

Fungicide performance for barley

• All diseases were better controlled in protectant situations
• For rhynchosporium and net blotch, prothioconazole and fluxapyroxad were effective
• For net blotch, fluopyram and bixafen (SDHIs) added activity
• For ramularia, mefentrifluconazole (in Revystar XE) added efficacy

Table 1. Products that featured in the 2021 fungicide performance presentation for wheat and barley.

*Arizona tested at full dose only. Folpet authorised for use in several products.

**Azoles.

***Strobilurins.

**** Recently authorised for use in barley.

Note: Some fungicides are tested as straights (solo) to determine performance but should be used in mixtures in practice – with at least one fungicide with an alternative mode of action that has efficacy against the target disease.

In the spray programme, make use of all modes of action (in sequence/mixture), as part of resistance management efforts.

Phoma stem canker

• Phoma controlled by azoles, SDHIs and strobilurins
• Average yield response 0.3 t/ha, with little benefit from applying more than 50% of full label rate (as part of two-spray programme)
• Some differences in canker control between products – however, yield differences were small (0.1–0.2 t/ha), especially when the disease index was less than 30

Light leaf spot

• Azoles and non-azoles provided similar levels of disease control and yield
• Some light leaf spot isolates have a decreased sensitivity to azoles (in laboratory tests), although field performance is not affected

Table 2. Products that featured in the 2021 fungicide performance presentation for oilseed rape.

*Products do not have a label recommendation for light leaf spot control but may be applied, at the appropriate time, for the control of other diseases.

**Recently registered.

***Also contains growth regulators (mepiquat chloride and prohexadione calcium).